An essential aspect of male fertility is the ability of sperm to actively move through certain regions of the female genital tract and penetrate the egg investments. Indispensable to these functions is a normal flagellum. Yet little is known about the genes which control sperm flagellar assembly or motility in mammals. The P.I. has mapped a locus, Hybrid Sterility-6 (Hst-6), to a region of equal to or less than 1 centimorgan (cM) in the proximal one third of mouse chromosome 17. The combination of certain alleles of this locus causes profound male sterility. Laboratory mice who are homozygous for an allele of Hst-6 (Hst-6S/Hst-6S) from another mouse species, Mus spretus (S), are sterile. Their sperm are immotile, due to the absence of a recognizable axoneme. Laboratory mice heterozygous for one copy of Hst-6S and a variant form of the proximal one third of chromosome 17 called a t haplotype (Hst-6S/t) are also sterile. Their sperm are motile, but the flagella of these sperm curve abnormally, in the same way as the flagella of sperm from sterile males carrying two t haplotypes (t/t). Thus, it is clear that Hst-6 has two genetically variant alleles, one affecting axonemal differentiation and the other, the curvature of the flagella of motile sperm. Because genes regulating these functions in mammals have not been isolated, the Hst-6 locus offers a novel approach to identify such genes. Therefore, the P.I. will first clone all testis-expressed genes that map to the Hst-6 locus (Aim I). To determine which of the candidate genes is most likely to be Hst-6, the P.I. will analyze the differences between the gene sequences and patterns of testicular mRNA and protein expression of each allele of each candidate gene with reference to our present knowledge of the Hst-6 mutant phenotypes (Aim II). Based upon the known functions of Hst-6 (axonemal assembly and flagellar curvature), it is likely that the Hst-6 protein specifically localizes in or very close to the axoneme of cauda epididymal sperm. The P.I. will test this hypothesis by using immunocytochemistry to examine cauda epididymal sperm for the specific subcellular location of candidate proteins that have already been localized to the tail region of developing spermatids and/or mature testicular sperm (Aim III). These experiments will contribute critical information to our understanding of the genetic control of sperm flagellar assembly and movement. Moreover, their success will produce a paradigm for the future study of the development and function of the mammalian sperm flagellum at the biochemical level.